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Photos:History's biggest mysteries

History's biggest mysteries – The earliest documented sighting of the mysterious creature swimming in Scotland's Loch Ness came in 1871, according to the monster's official website. Dozens of sightings have been logged since then, including the most recent in November 2011 when someone reported seeing a "slow moving hump" emerge from the murky depths of Loch Ness.

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Photos:History's biggest mysteries

History's biggest mysteries – The twin-masted merchant vessel Mary Celeste set sail from New York on November 7, 1872, bound for Genoa, Italy. Its 10 passengers were not on board when it was found floating in the Strait of Gibraltar four weeks later. There were no signs of a struggle, and all of the boat's cargo was still on board. Its only lifeboat was missing.

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Photos:History's biggest mysteries

History's biggest mysteries – The name Jack the Ripper came from a letter sent in 1888 to London police, purporting to be the killer of five prostitutes. His true identity has never been proved.

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Photos:History's biggest mysteries

History's biggest mysteries – The Shroud of Turin may be the most famous religious relic. Some Christians believe the shroud, which appears to bear the imprint of a man's body, to be Jesus Christ's burial cloth. The body appears to have wounds that match those the Bible describes as having been suffered by Jesus on the cross. Many scholars contest the shroud's authenticity, saying it dates to the Middle Ages, when many purported biblical relics -- such as splinters from Jesus' cross -- surfaced across Europe.

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Photos:History's biggest mysteries

History's biggest mysteries – The Lockheed Electra, piloted by Amelia Earhart and Fred Noonan, flies over the Golden Gate Bridge at the start of a round-the-world flight on March 17, 1937. The two vanished during a similar flight in 1937.

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Photos:History's biggest mysteries

History's biggest mysteries – A group of U.S. Navy Avenger planes, like the ones seen here in 1943, disappeared off the coast of southern Florida in December 1945. The planes were carrying 14 men. Thirteen more servicemen also vanished when they went to search for the missing fliers. That started the legend of the Bermuda Triangle. Other mysterious disappearances over the years have been linked to the Triangle, which is anchored by Bermuda, Florida and Puerto Rico.

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Photos:History's biggest mysteries

History's biggest mysteries – Stories of elusive giant, hairy human-like beasts have been told on several continents for centuries. In the Himalayan Mountains, it is known as the Yeti or the Abominable Snowman. North Americans have reported sightings of Big Foot or Sasquatch. Russians call theirs the Almasty.

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Photos:History's biggest mysteries

History's biggest mysteries – The Los Angeles County coroner ruled that actress Marilyn Monroe's death in this room was a "probable suicide" from an overdose of barbiturates. Despite the official conclusion, questions have lingered for decades about Monroe's death in August 1962. She was 36.

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Photos:History's biggest mysteries

History's biggest mysteries – U.S. President John F. Kennedy sits with the first lady in the back of a limousine November 22, 1963, in Dallas. Kennedy's assassination remains one of the most shocking events of the 20th century, and it's also one of its biggest mysteries. While a blue ribbon panel concluded there was only one gunman, a Gallup survey six decades later found that 60% of Americans don't believe that.

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Photos:History's biggest mysteries

History's biggest mysteries – Police sweep a field in Waterford Township, Michigan, in search of Jimmy Hoffa's body in July 1975. The ex-Teamsters boss was last seen that month outside a Detroit-area restaurant. His disappearance is a mystery that triggers occasional digs by FBI agents looking for his remains.

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Photos:History's biggest mysteries

History's biggest mysteries – Actors Robert Wagner and Natalie Wood are seen on their boat The Splendour in October 1976. Wood's body was found floating off California's Catalina Island in November 1981. It was initially ruled an "accidental drowning," but the Los Angeles County coroner officially changed the death certificate last year to read "drowning and other undetermined factors."

Such mysteries are "phenomenally gripping," Gray said. "Things that are unsolved just sort of grab people, especially when you have the common experience of flying."

Answers about what happen to the Boeing 777 and the people on board must come soon, he said. "When you get on the plane the next time, are you going to wonder a little bit about whether you are going to disappear?"

But history holds tight to some secrets, leaving us with just speculation, conspiracy theories and educated guesses. A mystery can have a long life, never forgotten and often re-examined.

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The assassination of U.S. President John F. Kennedy on November 22, 1963, remains one of the most shocking events of the 20th century. The shooting of his brother Robert F. Kennedy on June 5, 1968, added to the conspiracy theories. In each case, a lone gunman was accused. In both deaths, questions remain:

Was there a second shooter on a grassy knoll along JFK's motorcade route? How could a man with a mail-order rifle target the President in a moving car from such a distance? Lee Harvey Oswald was himself shot to death at a Dallas police station days after JFK's death.

A blue ribbon panel headed by Supreme Court Chief Justice Earl Warren concluded Oswald was the only assassin, but a Gallup survey six decades later found 60% of Americans don't believe that.

As for RFK, a witness told CNN in 2012 that she heard two guns firing during the 1968 shooting in Los Angeles and that authorities altered her account of the crime.

The mystery of Marilyn Monroe

Photos:The life and career of Marilyn Monroe

Photos:The life and career of Marilyn Monroe

The life and career of Marilyn Monroe – Marilyn Monroe, then 24, is photograhed in Los Angeles' Griffith Park in 1950. Sunday, August 5, marked the 50th anniversary of Monroe's death at age 36. See more from this series on LIFE.com.

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Photos:The life and career of Marilyn Monroe

The life and career of Marilyn Monroe – Marilyn Monroe's debut on the cover of LIFE Magazine, photographed by Philippe Halsman. See more from this series on LIFE.com.

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Photos:The life and career of Marilyn Monroe

The life and career of Marilyn Monroe – Marilyn Monroe and Jane Russell on the set of 1953's "Gentlemen Prefer Blondes." See more from this series at LIFE.com.

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Photos:The life and career of Marilyn Monroe

The life and career of Marilyn Monroe – Marilyn Monroe at age 22 in Hollywood during 1949. See more from this series on LIFE.com.

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Photos:The life and career of Marilyn Monroe

The life and career of Marilyn Monroe – Marilyn Monroe at home in Hollywood, 1953. See more from this series on LIFE.com.

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Photos:The life and career of Marilyn Monroe

The life and career of Marilyn Monroe – Marilyn Monroe at the time she filed for divorce from Joe DiMaggio in October 1954. See more from this series on LIFE.com.

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The Los Angeles County coroner ruled that actress Marilyn Monroe's August 1962 death was a "probable suicide" from an overdose of barbiturates. Despite the official conclusion, questions have lingered for decades about her death at 36.

The fuel for conspiracy theories include discrepancies about what time her body was found, the disappearance of her internal organs at the morgue and her links to President John F. Kennedy and his brother Robert F. Kennedy, as well as organized crime bosses.

Also, the Monroe the world saw was not the real Norma Jeane Baker. She was not a dumb blonde, but an intelligent, well-read brunette, according to those who knew her.

Was Natalie Wood's death an accident or murder?

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The death of actress Natalie Wood, whose was found floating off California's Catalina Island in November 1981, was initially ruled an "accidental drowning," but the Los Angeles County coroner officially changed the death certificate last year to read "drowning and other undetermined factors."

Homicide investigators decided to take a new look at one of Hollywood's most enduring mysteries after they were contacted by people who said they had additional information about the actress' drowning, the Sheriff's Department said.

Wood's death happened during a trip on her yacht with husband Robert Wagner and actor Christopher Walken.

"Since there are unanswered questions and limited additional evidence available for evaluation, it is opined by this medical examiner that the manner of death should be left as undetermined," the coroner said in a 2012 statement.

What happened to Amelia Earhart?

Photos:Photos: Amelia Earhart: Pioneer of the sky

Photos:Photos: Amelia Earhart: Pioneer of the sky

Amelia Earhart: Pioneer of the sky – Amelia Earhart laughs before a flight to Northolt, England, in a DH 82 Tiger Moth plane on June 24, 1928.

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Photos:Photos: Amelia Earhart: Pioneer of the sky

Amelia Earhart: Pioneer of the sky – Earhart poses in front of her plane in the 1930s.

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Photos:Photos: Amelia Earhart: Pioneer of the sky

Amelia Earhart: Pioneer of the sky – Earhart looks through the cockpit window of her plane.

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Photos:Photos: Amelia Earhart: Pioneer of the sky

Amelia Earhart: Pioneer of the sky – Earhart stands on the roof of the Hyde Park Hotel in London with Capt. Hilton H Railey on June 20,1928.

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Photos:Photos: Amelia Earhart: Pioneer of the sky

Amelia Earhart: Pioneer of the sky – A crowd surrounds Earhart, center, upon arrival at Hanworth airfield in London after she crossed the Atlantic Ocean in 1932.

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Photos:Photos: Amelia Earhart: Pioneer of the sky

Amelia Earhart: Pioneer of the sky – Earhart poses for a photo in Newfoundland, Canada.

Amelia Earhart: Pioneer of the sky – The Lockheed Electra 'Flying Laboratory' piloted by Earhart and Fred Noonan flies over the Golden Gate bridge in Oakland, California, at the start of a planned around-the-world flight on March 17, 1937. The trip had to be abandoned after the plane crashed on takeoff in Hawaii.

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Famed aviator Amelia Earhart vanished along with navigator Fred Noonan during a doomed attempt to fly around the world in 1937.

"There's still no concrete evidence as to what happened," Gray said.

Theories about her fate included one that Earhart's plane was forced down by the Japanese around the Marshall Islands. Another is that Earhart secretly returned to the United States and the government gave her a new identity.

Earhart was on the last segment of her global route, flying from Lae, Papua New Guinea, with a destination 2,500 miles away in the middle of the Pacific Ocean. If her plane crashed into the waters short of her planned refueling point on Howland Island, the wreckage is likely resting 17,500 feet below the ocean's surface.

Only in recent years has it been possible to explore those depths with underwater vehicles. Expeditions scanning the ocean's floor have offered tantalizing evidence in the past two years that Earhart's plane could be near Nikumaroro Island in the South Pacific.

Where is Jimmy Hoffa?

Photos:Photos: The mystery of Jimmy Hoffa

Photos:Photos: The mystery of Jimmy Hoffa

The mystery of Jimmy Hoffa – Nearly 40 years after his disappearance, former Teamsters boss Jimmy Hoffa, pictured circa 1955, remains among America's most famous missing persons. Authorities have been searching for the once powerful union boss since he vanished in 1975.

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Photos:Photos: The mystery of Jimmy Hoffa

The mystery of Jimmy Hoffa – Hoffa slumps in a chair at the Teamsters union office. He was one of the most powerful union leaders in America until being forced out of the organized labor movement. He went to prison in 1967 for jury tampering and fraud before being pardoned four years later.

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Photos:Photos: The mystery of Jimmy Hoffa

The mystery of Jimmy Hoffa – Hoffa appears at the Teamsters union convention in 1957, the year he first became union president.

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Photos:Photos: The mystery of Jimmy Hoffa

The mystery of Jimmy Hoffa – Hoffa, center, stands with other officials at the Teamsters convention, where he made a successful bid for control of the union in 1957.

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Photos:Photos: The mystery of Jimmy Hoffa

The mystery of Jimmy Hoffa – Hoffa testifies at a Senate Rackets Committee hearing in 1958.

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Photos:Photos: The mystery of Jimmy Hoffa

The mystery of Jimmy Hoffa – Hoffa on the phone at an airport in 1959.

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Photos:Photos: The mystery of Jimmy Hoffa

The mystery of Jimmy Hoffa – An office for Teamsters union local chapters that Hoffa set up.

The mystery of Jimmy Hoffa – Hoffa holds a Teamsters rally at Madison Square Garden in New York in 1960.

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Photos:Photos: The mystery of Jimmy Hoffa

The mystery of Jimmy Hoffa – Hoffa leads supporters at a Teamsters convention in 1959.

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Photos:Photos: The mystery of Jimmy Hoffa

The mystery of Jimmy Hoffa – The Teamsters boss appears on the cover of Life magazine on May 18, 1959. The headline reads, "A National Threat: Hoffa's Teamsters; Part 1: Sources of a Union's Uncurbed Power."

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Photos:Photos: The mystery of Jimmy Hoffa

The mystery of Jimmy Hoffa – Hoffa, pictured circa 1960, was a powerful labor leader at a time when unions wielded a great deal of sway over elections and were notoriously tied to organized crime.

The mystery of Jimmy Hoffa – Hoffa and his son, James Phillip, enter a federal courtroom in July 1964. His son is the current president of the Teamsters.

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Photos:Photos: The mystery of Jimmy Hoffa

The mystery of Jimmy Hoffa – Hoffa, second row, center, leaves court after being found guilty of jury tampering in 1964.

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Photos:Photos: The mystery of Jimmy Hoffa

The mystery of Jimmy Hoffa – Hoffa at the Pittsburgh airport in 1971 on his way back to federal prison after being let out to visit his ailing wife. He was released from prison later that year on the condition he not resume union activity before 1980.

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Photos:Photos: The mystery of Jimmy Hoffa

The mystery of Jimmy Hoffa – Hoffa poses for a picture on July 24, 1975, less than a week before his disappearance. He was 62 at the time.

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Photos:Photos: The mystery of Jimmy Hoffa

The mystery of Jimmy Hoffa – A Bloomfield Township, Michigan, police officer stands beside Hoffa's car after the former labor leader's disappearance in July 1975. Hoffa was last seen at a restaurant in suburban Detroit on July 30, 1975.

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Photos:Photos: The mystery of Jimmy Hoffa

The mystery of Jimmy Hoffa – Police sweep a field in Waterford Township, Michigan, in search of Hoffa's body in July 1975.

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Photos:Photos: The mystery of Jimmy Hoffa

The mystery of Jimmy Hoffa – Demolition workers tear down a horse barn for the FBI in 2006 in a search for Hoffa's remains in Milford, Michigan. The FBI had received a tip that Hoffa was buried on the farm.

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Photos:Photos: The mystery of Jimmy Hoffa

The mystery of Jimmy Hoffa – FBI agents search a field for Hoffa's remains on Monday, June 17, 2013, in Oakland Township, Michigan, outside Detroit. Alleged mobster Tony Zerilli tipped off the police, and a source close to the case said the information provided was "highly credible."

The FBI said at the time of his disappearance that it could have been linked to Hoffa's efforts to regain power in the Teamsters and to the mob's influence over the union's pension funds. Hoffa was thought to be trying to get back into a power position with the labor movement after his release from prison. He was sent to prison in 1967 for jury tampering and fraud. President Richard Nixon pardoned him in 1971.

Popular theories over the years include that Hoffa was disposed of under the old Giants Stadium in New Jersey, buried in a Michigan horse farm, taken to a Florida swamp as alligator food or that his body was incinerated. Another early theory was that Hoffa simply took off for South America with a go-go dancer.

Hoffa's middle name, by the way, is Riddle.

Who was Jack the Ripper?

The name Jack the Ripper came from a letter sent in 1888 to London police, purporting to be the killer of five prostitutes.

His true identity has never been proved. Stories about the serial killings on the streets of Victorian London captured readers around the world, spurring intense speculation that lasted well beyond a century.

Crime writer Patricia Cornwell contributed to the theories with her 2002 book "Portrait of a KIller -- Jack the Ripper: Case Closed." Cornwell fingers painter Walter Sickert, in part based on the similar watermarks on the letter to police and Sickert's personal writing paper. She also argued that many of his paintings depicted Jack the Ripper crime scenes.

The ghost ship Mary Celeste

The twin-masted merchant vessel Mary Celeste set sail from New York on November 7, 1872, bound for Genoa, Italy. Its 10 passengers were not on board when it was found floating in the middle of the Strait of Gibraltar four weeks later. There were no signs of a struggle, and all of its cargo was still on board. Its only lifeboat was missing.

Speculation over the past 140 years about why the ship was abandoned include theories involving pirates, a crew mutiny and even monster from the sea. A documentary titled "The True Story of the Mary Celeste" released in 2007 ruled out those possibilities but stopped short of a conclusive explanation.

What's the deal with the Bermuda Triangle?

The legend of the Bermuda Triangle began with the unexplained disappearance of a group of military planes carrying 14 men off the coast of south Florida in December 1945.

"We are entering white water, nothing seems right," the flight leader supposedly said before radio contact was lost. Thirteen more servicemen sent to search for the missing fliers also vanished.

Two British South American Airways passenger planes disappeared in the area a year apart in 1948 and 1949. No explanations or wreckage were ever found.

Victims credited to the area in numerous books and documentaries have included a large oil tanker, a pleasure yacht and a small passenger plane. Books, including "The Devil's Triangle," "Limbo of the Lost," and "The Riddle of the Bermuda Triangle," suggest supernatural explanations. Aliens in spaceships, wormholes and the mythical lost continent of Atlantis have been blamed.

Are Big Foot, Sasquatch or Yeti for real?

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Stories of elusive giant, hairy human-like beasts have been told on several continents for centuries. In the Himalayan Mountains, it is known as the Yeti or the Abominable Snowman. North Americans have reported sightings of Big Foot or Sasquatch. Russians call theirs the Almasty.

One common feature in modern reports is that most photos are blurry and video is shaky. Serious examination often leads to conclusions of mistaken identity or elaborate hoaxes.

But just last year, a British geneticist said that hair samples supposedly from two of the mystery creatures proved to be a genetic match to an ancient polar bear. The scientist submitted his DNA results for publication in a peer-reviewed science journal, and it is due to publish a book based on his research this year.

Is the Loch Ness Monster a real creature?

The earliest documented sighting of the mysterious creature swimming in Scotland's Loch Ness came in 1871, according to the monster's official website. Yes, it has a website that serves the purpose of promoting tourism to the area. Dozens of sightings have been logged since then, including the most recent in November 2011 when a Mr. George Edwards reported seeing a "slow moving hump" emerge from the murky depths of Loch Ness.

A U.S. research team targeted Nessie -- the creature's affectionate nickname -- in 2009 using a submarine to explore the lake's bottom. The only discovery were thousands of golf balls 300 feet down and 100 yards off the shore.

No mystery there, though: Locals and tourists have been known to practice their golf swing there for years.

Was the Shroud of Turin the burial cloth for Jesus?

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The Shroud of Turin may be the most famous religious relic.

Some Christians believe the shroud, which appears to bear the imprint of a man's body, to be Jesus Christ's burial cloth. The body appears to have wounds that match those the Bible describes as having been suffered by Jesus on the cross.

Many scholars contest the shroud's authenticity, saying it dates to the Middle Ages, when many purported biblical relics -- such as splinters from Jesus' cross -- surfaced across Europe. Even the Roman Catholic Church does not insist the shroud was used to wrap the body of Jesus. Its official position is that the shroud is an important tool for faith regardless of its authenticity.